There is currently a rule in the industry that laser equipment supplied with an indicator lamp use a wavelength other than that of the laser, so that an operator wearing protective eyewear will be able to see the indicator lamp when the laser is on. As a consequence, previous arrangements wherein a small portion of the laser light itself is leaked out for indicator purposes are no longer acceptable.
Candidate systems include optical stimulated-emission arrangements wherein laser light is used to illuminate a sample under investigation. The prior-art system of FIG. 1 shows the delivery of an excitation laser beam to a sample under test 200 by way of an excitation fiber-optic cable 202. Scattered light from the sample is collected by the probe head and routed back to the analyzer via a separate collection fiber-optic cable 204. The non-shifted or Rayleigh line is removed from the collected scatter by a notch filter 210 supported in the collection path, and a beam combiner 212 serves to combine the laser beam delivery path onto a common optical axis with the collection path, so that a common sampling optic 214 may be used for both paths.
In this particular configuration, a grating 206 and spatial filter 208 are inserted into the beam delivery path to remove the scattering signature of the fiber itself. Conveniently, since some of the excitation leaks through the grating 206, this stray radiation may be used as an indicator, in this case by striking diffuser 207 mounted on the probe head enclosure. However, as explained above, since this indicator light is identical to that of the primary beam, use of modem, efficient laser protection eyewear will prevent an operator from knowing whether the laser has been switched on. There is an outstanding need for solution to this problem, preferably an inexpensive solution involving a wavelength other than that of the laser to reliably indicate the present/absence of a laser beam which might otherwise be harmful to an operator unless protective eyewear is used.